What lab tests are needed to confirm the diagnosis of Inflammatory Bowel Disease (IBD) in a patient intolerant to corticosteroids?

Laboratory Tests to Confirm IBD Diagnosis

To confirm the diagnosis of inflammatory bowel disease, you must obtain routine blood tests (complete blood count, CRP, albumin, liver function, iron studies, renal function, vitamin B12), fecal calprotectin (with a threshold of 100 μg/g providing optimal diagnostic accuracy at 93% sensitivity and 96% specificity), and stool cultures to exclude infectious causes—but these laboratory tests alone cannot establish the diagnosis and must be combined with ileocolonoscopy with biopsies from at least five sites and cross-sectional imaging. 1, 2, 3

Essential Laboratory Panel

Blood Tests

• Complete blood count to assess for anemia, which is common in IBD 1, 3
• C-reactive protein (CRP) as the primary inflammatory marker, though recognize that approximately 20% of patients with active Crohn's disease may have normal CRP levels 1, 3
• Albumin to evaluate nutritional status and disease severity 1
• Liver function tests as part of baseline assessment 1, 3
• Iron studies (serum ferritin and transferrin saturation), interpreting ferritin <30 μg/L as iron deficiency without inflammation, or up to 100 μg/L as potentially indicating deficiency in the presence of active disease 3
• Renal function for baseline values 1
• Vitamin B12 levels, particularly important in Crohn's disease with ileal involvement 1

Stool Studies

• Fecal calprotectin with a cut-off of 100 μg/g (sensitivity 95%, specificity 91% for differentiating IBD from non-IBD diagnoses) 1, 2
• Stool cultures for bacterial pathogens 2, 3
• Clostridium difficile toxin assay to exclude infectious mimics before finalizing the diagnosis 4, 2, 3
• Ova and parasites testing 2

Critical Limitation of Laboratory Tests

Laboratory tests cannot diagnose IBD independently—they serve only as supportive evidence and must be integrated with endoscopic findings, histology, and imaging. 2, 3 The diagnosis requires a systematic combination of clinical assessment, laboratory testing, endoscopic evaluation with histology, and radiological imaging. 2

Required Confirmatory Procedures Beyond Laboratory Tests

Endoscopic Evaluation (Mandatory)

• Complete ileocolonoscopy examining the terminal ileum and all colonic segments is essential for diagnosis 1, 4, 2
• Multiple biopsies from at least five sites (terminal ileum, cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum), including from normal-appearing mucosa, stored in separate containers 4, 2
• Biopsies from unaffected areas are essential to document histologically the spared segments between inflammatory areas, which is characteristic of Crohn's disease 1, 4

Cross-Sectional Imaging (Mandatory)

• MRI or CT enterography is needed in all patients at diagnosis to assess disease extent and severity, particularly for small bowel involvement that cannot be detected by colonoscopy 1, 4, 2

Common Pitfalls to Avoid

• Do not rely solely on laboratory markers for diagnosis, as no single test is specific or sensitive enough for IBD 3
• Do not exclude active disease based on normal CRP alone, since 20% of patients with active Crohn's disease have normal levels 1, 3
• Do not make an IBD diagnosis without excluding infectious causes first, as this leads to inappropriate treatment 3
• Do not use serological markers (pANCA, ASCAs) to differentiate between Crohn's disease and ulcerative colitis, as their accuracy is limited and they are ineffective at distinguishing colonic Crohn's from ulcerative colitis 3
• Do not interpret iron studies without considering inflammation status, as this can lead to missed iron deficiency diagnosis 3

Interpretation of Fecal Calprotectin Results

• <100 μg/g: Lower probability of relapse 2
• 100-200 μg/g: Warrants repeat testing 2
• >200 μg/g: Triggers endoscopic evaluation 2
• <250 μg/g: Associated with endoscopic and histological remission and protection against hospitalization or colectomy 2